In manufacturing a seamless steel pipe or tube (hereinafter often refers to as “pipe”), the Mannesmann mandrel mill system, Mannesmann plug mill system, Mannesmann-Assel mill system, or the like is used to pierce and roll a billet with a round section using a piercing mill. The methods for manufacturing a seamless pipe by these Mannesmann systems include:
piercing and rolling a billet, heated to a predetermined temperature in a heating furnace, using an inclined roll type piercing mill;
expanding and decreasing the radial thickness of the resulting hollow pipe stock using a elongator such as a mandrel mill and a plug mill;
narrowing the outer diameter of the hollow pipe stock using a rolling mill such as stretch reducer and sizing mill, resulting in a finished steel pipe product.
In regard to the production methods of a seamless steel pipe, various inventions are disclosed.
For example, patent document 1 discloses an invention related to manufacturing a pipe stock for a seamless steel pipe by rolling and piercing a high-Cr alloy steel billet containing Cr at 9% or more by weight using a piercing mill after heating the billet in a heating furnace, characterized in rolling the billet in such a condition that δ-ferrite does not occur in the metal structure of the pipe stock. An Example of patent document 1 describes manufacturing a seamless steel pipe from a 12% Cr steel.
Patent document 2 discloses an invention related to piercing and rolling a seamless steel pipe made of high carbon steel containing, by mass percent, C of 0.95 to 1.10% after billet heating, characterized in that the billet-heating temperature is set at 1200° C. or less, and an average strain rate εAV in the axial direction is set at 2.0 sec−1 or less. An Example of patent document 2 describes producing a seamless steel pipe from a steel equivalent to SUJ-2.
In recent years, oil country tubular goods, boiler pipes, and the like are used in increasingly harsh environments. For this reason, properties required to the seamless steel pipes used for the pipes are becoming highly demanding. The pipes used in oil wells which are becoming much deeper and much more corrosive environments. Higher strength and superior corrosion resistance are required the pipes. Superior corrosion resistance in high temperature pure water and hot water including Cl−, especially superior stress corrosion cracking resistance are required to the pipes used in nuclear-power generating facilities, chemical plants, and the like. Therefore, seamless steel pipes, made of high alloy steels containing large quantities of Cr, Ni, and further Mo, are becoming popular.
[Patent document 1] JP H10-180312A
[Patent document 2] JP 2001-137913A